Various techniques have previously been used to preheat molding compound in the form of resin pellets prior to injecting the molding compound into a mold cavity containing an integrated circuit. Initially, the resin pellets often have the general configuration of an elongated cylinder with dimensions selected to allow placing a preheated pellet into a mold pot. Generally, the mold pot includes a bushing which holds the preheated pellet while a transfer ram applies force to the pellet to inject molding compound into a mold cavity. Typically, a mold assembly including the mold pot and a mold die having main runners, secondary runners, mold gates, and multiple mold cavities is used to encapsulate integrated circuits. The portion of the mold die containing the mold cavities is sometimes referred to as the chase or mold chase.
Some preheat methods employ upper and lower electrodes which are spaced parallel to each other and preheat a resin pellet such that both ends have a higher temperature than the center of the pellet. When the preheated pellet is placed in the mold pot bushing and contacted by the transfer ram, the pellet will deform with the general cross-section of an hour glass or figure 8 having voids between the pellet and bushing. These voids often trap air or other gases as the transfer ram continues to apply force to the pellet and inject molding compound into a mold cavity.
Another technique for preheating a resin pellet uses a stepped plate in the center of the upper electrode to increase the temperature in the middle of the pellet. The temperature at both ends of the preheated pellet will then be less than the temperature at the center of the preheated pellet. This increased center temperature produces a preheated pellet having the general cross-section of a beer barrel when the transfer ram contacts the preheated pellet in a mold pot bushing. Again, void spaces result which often trap air or other gases as the transfer ram applies force to the pellet to inject molding compound into a mold cavity.
Prior molding techniques have generally worked satisfactorily in packaging previous integrated circuit designs. With the advent of more complex and smaller scale integrated circuits, increased importance has been placed on uniform encapsulation of the integrated circuit and its associated electrical components. Voids or trapped air in the molding compound frequently result in rejection of the finished product which substantially reduces the yield of a manufacturing facility producing complex integrate circuits.
A need has thus arisen for a molding process which properly preheats resin pellets to produce a uniform, consistent flow of molding compound throughout the molding process without entrapping air or other gases.